Samples of rodent tumor or normal tissues from National Toxicology Program carcinogenicity studies, from in-house carcinogenicity studies, and from various other contract efforts continue to be collected and frozen for subsequent molecular biology studies. To date over 10,000 samples have been frozen for use by various NIEHS investigators and approximately 1,000 of these samples have been or are in the process of being used for oncogene or suppressor gene studies. Results from analysis of these samples is yielding useful information that will help in interpretation of carcinogenicity bioassays, in understanding the process of carcinogenesis, and in assessing human health risk. It is anticipated that collection efforts will continue over the next three years.